Biochar-Iron Oxide Composites for Adsorption of Chlorophenoxy Herbicides: Impact of Chlorine Substituents on Bath and Continuous Adsorption Performance

Abstract

Iron oxide-impregnated biochars are synthesized from sugarcane bagasse using FeCl3 as a modifier and pyrolyzed at 400 °C (Fe400) or 600 °C (Fe600) to enhance adsorption of 2,4-dichlorophenoxyacetic acid (2,4-D) and chlorophenoxyacetic acid (4-CPA). Fe600 exhibits increased graphitic structures, confirmed by infrared and Raman spectroscopy, with X-ray diffraction identifying magnetite and hematite crystalline phases within an amorphous carbon matrix. N2 adsorption analysis indicates a reduced specific surface area for Fe600 (241 m2 g−1) due to decreased micropore volume at higher pyrolysis temperatures. Adsorption tests demonstrate Fe600's stable performance across varying pH and ionic strengths, achieving maximum capacities for 2,4-D and 4-CPA of 30.9 and 40.3 mg g−1, respectively. Density functional theory (DFT) calculations indicate favorable interactions between 2,4-D and Fe600's surface, constrained by smaller pores. Iron leaching from Fe600 remains within safe limits established by World Health Organization standards. Real water matrix tests validate Fe600's robust adsorption capacity. Fixed-bed column experiments show reduced mass transfer resistance and prolonged service life, achieving a maximum bed adsorption capacity of 19.66 mg g‒1 for 4-CPA. These findings underscore the influence of chlorine atoms in chlorophenoxy herbicides adsorption and confirm Fe600 as an effective adsorbent for 2,4-D remediation in contaminated water.